finding a link or material pointing to a missing file or path
In the outliner choose the Orphan Data display mode

To fix the path for broken links or missing items use the Data Api mode

Data API
Lists every data-block along with any properties that they might have.
Orphan Data
Lists data-blocks which are unused and/or will be lost when the file is reloaded. It includes data-blocks which have only a fake user. You can add/remove Fake User by clicking on cross/tick icon in the right side of the Outliner editor.

(from the blender manual)